Leah here from leah4sci.com and in this video I'm going to show you the pencil trick to
quickly and easily identify primary, secondary, tertiary, quaternary, carbon and other atoms.
Let's start with carbon atoms.
When you're looking at a molecule if you see a carbon atom bound to other atoms, the way
you classify this carbon is by looking at how many carbons are attached to it.
In the first example I give you methane.
It doesn't have any carbons attached so it wouldn't fall under this classification.
But if I take this carbon, remove one hydrogen atom and swap that for a CH3, this carbon
is now considered a primary carbon atom cuz it's attached to one of our carbon atom.
If I remove 2 hydrogen atoms and replace them with other carbons for example methyl groups,
the central carbon now being attached to two other carbon atoms is considered a secondary
carbon.
If I swap three hydrogens for carbon in this case methyl, the central atom is now attached
to 1,2,3 carbon atoms making it a tertiary carbon.
And last but not least, if I swap each of the hydrogens for a carbon atom in this case
4-methyls, that central carbon is attached to 1,2,3,4 carbon atoms making it a quaternary
carbon.
This is the simple approach, but what happens if the molecule you see is not drawn in lewis
structure?
What if you're not shown a lewis structure where all the carbons are visible?
What if you have to figure this out in line structure?
Do you draw it out so that you can clearly see what's going on?
Absolutely not!
This is where we introduce the pencil trick and this is how it works.
Imagine that my stylist is the pencil and you put it down on the carbon atom.
What you wanna do is look at that pencil point and see what is coming out of that pencil
point specifically we're looking for bonds to carbon atoms.
If I want to know the substitution of this carbon here, all I do is put down my pencil
and look at how many lines come directly from that point.
Right now I see 1, 2, 3.
3 lines coming out of it means 3 carbons attached making this a tertiary carbon.
What about this one right here?
I only have one line coming out of my pencil tip making it primary.
You'll notice that all terminal carbons are going to be primary so anytime you see the
end of the chain knowing that this works once, it will always work, these are all primary
carbons.
What about this one right here?
From my pencil tip I see 1,2 making that a secondary carbon atom.
This trick works anytime you need to classify carbon atoms not just in a standard boring
chain.
For example say you have a molecule with a carbocation and you're trying to determine
which carbocation is more stable.
What do we know about carbocation stability?
The more substituted the more stable as explained in the video linked below.
So how do we figure this out?
Your pencil down, we have two bonds coming out of it, this right here is secondary carbocation.
On the right put your pencil down, one line coming out of it, this is a primary carbocation.
Same trick applies to radicals.
The molecule on the left has a radical at the end, if we put the pencil down, one line,
one bond to carbon making this a primary radical.
And the molecule on the right when you put the pencil down has two lines coming out of
it meaning two bonds to carbon making it a secondary radical.
When it comes to an atom attached to carbon we look at it slightly differently, we're
still looking at the carbon atom but specifically the carbon holding that atom in question.
Say I ask you how many primary hydrogens we have in this molecule?
We can't look for the bonds coming from hydrogen to carbon because as you know hydrogen can
only form one bond and in this case it'll be one bond between carbon and hydrogen.
Instead, we're looking for the number of primary carbons and then figuring out how many hydrogens
are on them.
What we need is to find which carbon atom has just one line coming out of it to make
it a primary carbon atom and that gives us a total of 3, then figure out how many hydrogens
sit on this carbon.
The trick to figure this out is 4, being the total number of bonds you can have on carbon,
minus all of the visible bonds because these are visible to other atoms like carbon hydrogen,
oxygen, nitrogen.
The rest will be hydrogen atoms.
For this carbon, we see one bond, four minus one equals to three.
And we can prove it by drawing it in, now I'm only drawing this to show you but on exams
you'll never ever wanna waste time drawing in hydrogen atoms.
Carbon below same thing, four minus one is three, and carbon on the left, four minus
one is three, We have three primary carbons, each containing three hydrogens for a total
of nine primary hydrogen atoms.
For secondary hydrogens, we're looking for secondary carbons, which when you put the
pencil down has two lines coming out of it and we just have one.
But how many hydrogens sit on that carbon?
If we have four bonds, we have four possible bonds minus the two visible to carbon equals
two hydrogen atoms on a secondary carbon and we have one secondary carbons which gives
us two secondary hydrogens.
For tertiary hydrogen, we're looking for a tertiary carbon, that's carbon with three
lines coming out of my pencil tip and we have one of them.
Carbon can have 4 bonds minus 3 bonds to carbon is 1 remaining for hydrogen, which means 1
tertiary carbon having one hydrogen atom gives us a total of one tertiary hydrogen.
We can't have quaternary hydrogen because if carbon can have a max of 4 bonds and those
are all bonds to carbon we have nowhere left for the hydrogen.
Same thing works for alcohol, we're looking at the degree of substitution of the carbon
atom specifically that is holding your alcohol.
For example, if I give you methanol, methanol would not be considered a substituted alcohol
because the carbon that is holding the oxygen is not bound to any other carbon atom so if
I put my pencil down I have no lines to carbon, that means it's not substituted at all.
If we look at ethanol, the alcohol is attached to this carbon, there is one line to another
carbon, remember it's a line to carbon, we do not count the line to oxygen because we're
looking specifically at the carbon holding the oxygen.
How many carbons is that attached to?
Ethanol would be a primary alcohol.
If we look at isopropyl alcohol, the central carbon is holding on to the oxygen but it
also got two lines coming out to carbon atoms making this a secondary alcohol.
Tert-butanol or tertiary butyl alcohol has three lines coming out of my pencil tip making
this a tertiary alcohol.
What if I want a quaternary alcohol?
Can I do that?
I would attempt to draw the molecule backwards, I would start with my carbon atom with my
pencil point.
Attempt to draw four bonds to carbon but then I can't put it in alcohol because that oxygen
is bound to a carbon that already has four bonds, that means we have 5 bonds to carbon
which is not allowed, it violates the octet.
So we have to stop at tertiary alcohols.
Alkyl halides are classified the same way.
Say i have a molecule with multiple chlorine atoms and I want to identify the degree of
substitution of each one.
We'll start with the purple coin on the right, putting the pencil down, I have just one line
, one bond to carbon making it a primary halogen or a primary alkyl-halide.
The green chlorine sits on the carbon that has three lines coming out of it.
Three bonds to carbon making it a tertiary chloride or a tertiary alkyl-halide.
Finally the blue chlorine sits on a carbon that has two lines, two bonds to carbon coming
out of it making it a secondary chloride.
The last type of molecule I wanna look at are substituted amines.
Say I'll give you a molecule that looks like this, We have a CH3 bound to a Nitrogen, bound
to an isopropyl group.
When looking at the carbon holding the nitrogen am I using the the one on the left or am I
using the one on the right?
The answer is both and neither.
Amines are different, we're not looking at the substitution of the carbon holding the
nitrogen, instead we're looking at the number of carbon atoms coming out of nitrogen, that
means the pencil trick applies to the nitrogen directly and this would be considered a secondary
amine, not because the carbon is bound to two carbons but because the nitrogen is bound
to two carbons.
So let's classify amines starting with ammonia.
Ammonia is not an amine because it is not bound to any carbon atoms, remember the amine
functional group is an r-bound to an N where R can be any sort of carbon chain.
The simplest amine would be a nitrogen bound to two hydrogen atoms and one carbon, for
example a CH3.
This Nitrogen is bound to one carbon making it a primary amine.
If I remove two hydrogens and swap them for R-groups for example 2 methyls, the Nitrogen
when you put your pencil down on Nitrogen you get two lines coming out of it towards
carbon, Nitrogen is bound to two R-groups making this a secondary amine.
If I replace all 3 Hydrogen atoms with carbon, when I put my pencil down I get 1,2, 3 lines
coming out of Nitrogen to give me a tertiary amine and that's it!
That's all the hydrogen atoms to replace.
That doesn't mean I stop there.
Remember ammonia could be a base and a lone pair can grab a hydrogen which means, if I
swap all three hydrogen atoms and the lone pair for an R-group for example 4 methyls.
First thing I have to remember is the formal charge.
Should minus has as thought in the video linked below, in this case would be 4 minus 3 which
is equal to plus 1, this would be a quaternary amine which is actually called an ammonium
where the ium reminds you that it has a positive on the Nitrogen.
Let's put all of this together by identifying the primary, secondary, and tertiary carbons
as well as the substitution of the alcohols and nitrogen on this molecule.
This is the molecular structure of Morphine which I found on google.
Let's start high to low, how many quaternary carbons do we have on this molecule?
We're looking for anywhere I can put my pencil down and get 4 lines coming out of it.
For example, this right here has two that's not quaternary, this one has 3, not quaternary.
We're looking for carbon with 4 bonds which would be this one right here, and that's it.
Moving down to tertiary, we're looking for carbon with 3 bonds coming out of it.
This carbon has three bonds coming out to carbon, that's tertiary for 1,2,3, this one
looks tertiary but that a bound to Nitrogen not carbon, doesn't count.
This one, same thing, has a bound to Oxygen, doesn't count.
Same thing bound to Oxygen, same thing, and same thing.
Looks like we have just three tertiary carbons.
Let's move down to secondary carbon atoms.
Now we have the ones that appear to be tertiary but one of the bonds is to a non carbon which
a heteroatom like Oxygen or Nitrogen so we have 1,2,3,4,5,6,7,8,9, ops! that's not secondary
coz it's a bond to Nitrogen. 10,11, it's 11.
Next we move on to primary carbons and we have, that's not a primary carbon because
it is not bound to another carbon atom.
This would be a primary carbon because it's bound to carbon and nitrogen.
We have just one primary carbon.
Now let's look at our alcohols.
For the alcohols, we have this Oxygen sitting on a secondary carbon as is this oxygen that
gives us two secondary alcohols and the amine remember has its own classification.
We're looking at the number of bonds coming out of the Nitrogen atom, that would be 1,2,3,
per one tertiary amine.
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